OBJECTIVE: To examine and diagnose the etiology of spontaneous cases of fatal lymphoproliferative disease in a laboratory colony of common marmosets (Callithrix jacchus)/ RESULTS Using PCR technology, we diagnosed a novel gammaherpesvirus in this colony. Sixteen cases of fatal lymphoproliferative disease were diagnosed in a laboratory housed population of 230 common marmosets (Callithrix jacchus) at the Wisconsin Regional Primate Research Center from 1992 to 1996. This represented 20% of the non-research related deaths of adult marmosets. Animals presented with a history of weight loss, inappetence, variable diarrhea, and in several cases palpable abdominal masses. Mesenteric lymph nodes and intestinal mucosa were consistently infiltrated by a mixed population of lymphocytes, obliterating the normal architecture. Two animals had lymphocytic leukemia. Affected animals ranged from 16 months to 9 years and there was no gender prevalence. There is no significant familial relationship among the affected animals. Eighty-four animals in the colony were tested for titers to Epstein-Barr virus and 52 were positive. Eight animals were tested for titers to Herpesvirus saimiri and all were negative. Polymerase Chain Reaction (PCR) yielded a gammaherpesvirus closely related to but distinct from Epstein-Barr virus and Herpesvirus papio. FUTURE DIRECTIONS To further study this gammaherpesvirus and to attempt to fill Koch's postulate. KEY WORDS gammaherpesvirus, lymphoproliferative disease, Callithrix jacchus